The thing is, if you could and nodes to the top of a slab and adjust the heights, it would no longer be a slab (no parallel surface top and bottom no longer horizontal - which I know is what you are asking for).
That means every time you add a node the slab would have to automatically split into separates sections for each plane (surface) - I am not sure that would be easy to program.
However, use a mesh as that is the best tool to adjust the top surface.
Now you have 2 choices.
1. use the 'Create Roofs from Mesh' add-on (part of the 'Goodies' add-ons) and that will create separate roofs for each slope - exactly what you want.
2. Copy and paste the mesh in exactly the same position.
Select one of the meshes and elevate it down the distance you want.
Perform a Solid Element Operation to subtract the lower mesh from the upper mesh.
You will have a mesh with parallel surfaces.
The advantage of 2 is it will still be a mesh, so is easy to edit further.
You just have to delete the lower mesh, copy & paste, lower and SEO again if you do make adjustments.
If using method one, you need to keep a copy of the original mesh in a hidden layer if you want to edit it again.
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